/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.giraph.utils;

import java.lang.reflect.Array;
import java.lang.reflect.GenericArrayType;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.lang.reflect.TypeVariable;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * Helper methods to get type arguments to generic classes.  Courtesy of
 * Ian Robertson (overstock.com).  Make sure to use with abstract
 * generic classes, not interfaces.
 */
public class ReflectionUtils {
    /**
     * Get the underlying class for a type, or null if the type is
     * a variable type.
     *
     * @param type the type
     * @return the underlying class
     */
    public static Class<?> getClass(Type type) {
        if (type instanceof Class) {
            return (Class<?>) type;
        }
        else if (type instanceof ParameterizedType) {
            return getClass(((ParameterizedType) type).getRawType());
        }
        else if (type instanceof GenericArrayType) {
            Type componentType =
                ((GenericArrayType) type).getGenericComponentType();
            Class<?> componentClass = getClass(componentType);
            if (componentClass != null ) {
                return Array.newInstance(componentClass, 0).getClass();
            }
            else {
                return null;
            }
        }
        else {
            return null;
        }
    }

    /**
     * Get the actual type arguments a child class has used to extend a
     * generic base class.
     *
     * @param baseClass the base class
     * @param childClass the child class
     * @return a list of the raw classes for the actual type arguments.
     */
    public static <T> List<Class<?>> getTypeArguments(
            Class<T> baseClass, Class<? extends T> childClass) {
        Map<Type, Type> resolvedTypes = new HashMap<Type, Type>();
        Type type = childClass;
        // start walking up the inheritance hierarchy until we hit baseClass
        while (! getClass(type).equals(baseClass)) {
            if (type instanceof Class) {
                // there is no useful information for us in raw types,
                // so just keep going.
                type = ((Class<?>) type).getGenericSuperclass();
            }
            else {
                ParameterizedType parameterizedType = (ParameterizedType) type;
                Class<?> rawType = (Class<?>) parameterizedType.getRawType();

                Type[] actualTypeArguments =
                    parameterizedType.getActualTypeArguments();
                TypeVariable<?>[] typeParameters = rawType.getTypeParameters();
                for (int i = 0; i < actualTypeArguments.length; i++) {
                    resolvedTypes.put(typeParameters[i],
                                      actualTypeArguments[i]);
                }

                if (!rawType.equals(baseClass)) {
                    type = rawType.getGenericSuperclass();
                }
            }
        }

        // finally, for each actual type argument provided to baseClass,
        // determine (if possible)
        // the raw class for that type argument.
        Type[] actualTypeArguments;
        if (type instanceof Class) {
            actualTypeArguments = ((Class<?>) type).getTypeParameters();
        }
        else {
            actualTypeArguments =
                ((ParameterizedType) type).getActualTypeArguments();
        }
        List<Class<?>> typeArgumentsAsClasses = new ArrayList<Class<?>>();
        // resolve types by chasing down type variables.
        for (Type baseType: actualTypeArguments) {
            while (resolvedTypes.containsKey(baseType)) {
                baseType = resolvedTypes.get(baseType);
            }
            typeArgumentsAsClasses.add(getClass(baseType));
        }
        return typeArgumentsAsClasses;
    }
}
